In a battery or a cell utilizing a lithium containing electrode it is important to eliminate as many impurities as possible which may affect cell performance. Such impurities include water. More particularly, the rechargeability of a lithium metal foil electrode is limited by side reactions between metallic lithium and impurities, especially water. When water reacts with lithium there is formed a solid surface layer on the lithium which increases the impedance of the anode (negative electrode). Such surface layer is formed by consuming the active material lithium metal.
The surface layer formed by the reaction between lithium and water will have a detrimental effect on the subsequent stripping and replating of metallic lithium during cycling of the battery. These effects are most likely to be observed as relatively rapid formation of dendritic lithium powder. The reaction which forms dendritic lithium powder isolates the lithium powder both physically and electrically from the metallic anode body whereby the dendritic powder becomes electrochemically inactive for further use as an active material in the cell. The result is that the life cycle of the battery is reduced due to such undesired interaction with water. Such interactions may also occur when a lithium alloy or lithium containing compound is used. The source of water which causes such adverse reaction, may be from any component within the cell including components of the negative electrode, the positive electrode and the electrolyte. Therefore, prior to assembly of the completed cell, it is preferred that all water be eliminated. However, this is very difficult and often nearly impossible.
Although it is possible to remove water from the liquid electrolyte precursor components, such as by separation of liquid electrolyte precursors from water over molecular sieves, it is nearly impossible to eliminate or remove water from precursor electrode pastes, which are precursors to the active material of the anode (negative electrode) and cathode (positive electrode). Such electrode pastes contain carbon, active material and a polymer which is typically a binder or a combination of binder and electrolyte. These materials typically have high viscosities and cannot be simply dried by passage over molecular sieves. Typical active materials for electrodes include intercalation compounds, such as metal oxides, which releasably retain lithium during cyclic operation of the cell. The precursor paste contains volatile components such as precursors of the polymer binder electrolytes which are needed in the final product, and which undergo a solidification process and a polymerization process and cannot be heated prematurely. Therefore, it is not possible to simply remove water from the pastes by heating the precursor paste to a temperature approaching 100.degree. C., the boiling point of water.
Therefore, what is needed is a method for removing water from constituents of an electrochemical cell or battery, and particularly from electrode compositions and precursors therefor.